A virtual machine (VM) is a portion of software that, when executed on appropriate hardware, creates an environment allowing the virtualization of an actual physical computer system. Each VM may function as a self-contained platform, running its own operating system (OS) and software applications (processes). An OS is a set of programs that manages computer hardware resources and provides common services for application software. For hardware functions such as input and output and memory allocation, the OS typically acts as an intermediary between application programs and hardware. OSs may be found on a variety of devices, including desktop computers, servers, smartphones, video game consoles, and so forth.
A host machine (e.g., computer or server) is typically enabled to simultaneously run one or more VMs, where each VM may be used by a local or remote client. The host machine allocates a certain amount of the host's resources to each of the VMs. Each VM may use the allocated resources to execute applications, including an OS known as a guest OS or guest. The hypervisor virtualizes the underlying hardware of the host machine or emulates hardware devices, making the use of the VM transparent to the guest or the remote client that uses the VM. Typically, a hypervisor manages allocation and virtualization of computer resources and performs context switching, as may be necessary, to cycle between various VMs.
The Advanced Configuration and Power Interface (ACPI) specification provides an open standard for device configuration and power management by the OS. The ACPI specification defines four global “Gx” states G0 through G3, six sleep “Sx” states S0 through S5, four device states D0 through D3, four processor states C0 through C3, and up to 17 implementation-dependent power-performance states P0 through Pn, where n is an integer between 1 and 16 inclusive. In addition, ACPI defines several tables that provide an interface between an ACPI-compliant OS and system firmware, including a Differentiated System Description Table (DSDT), a Secondary System Description Table (SSDT), and a System Resource Affinity Table (SRAT).